Petrol vs Diesel

Spark ignition versus compression ignition, and what changes for the technician.

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What you will learn

Explain the difference between SI and CI combustion
Describe petrol injection versus diesel injection pressures
Identify the main emissions devices for each

A petrol engine is spark-ignition (SI): it mixes air and fuel, compresses it and ignites it with a spark plug. A diesel engine is compression-ignition (CI): it compresses air alone to a very high pressure and temperature, then injects fuel that ignites on contact.

The practical consequences are large. Diesel injection pressures can exceed 2,000 bar in common-rail systems, so high-pressure lines are dangerous and require strict cleanliness. Petrol engines control load by throttling air; diesels largely control load by fuel quantity.

Typical example for training: a small common-rail diesel may idle at 250–350 bar rail pressure and exceed 1,600 bar at full load, but exact values depend on the manufacturer and system generation.

Key points

Wrong fuel has very different consequences for each type.
Diesel needs clean, high-pressure fuel and robust glow assistance.
Emissions hardware (EGR, DPF, SCR) dominates modern diesel faults.

Key points

Petrol ignites with a spark; diesel ignites from compression heat
Diesel injection pressures are far higher
Both use O2 sensors, catalysts and particulate control

All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.